A known braking apparatus for a vehicle such as disclosed in JP2006-21745A includes a hydraulic brake apparatus and a regenerative brake apparatus. The hydraulic brake apparatus generates a basic hydraulic pressure by a master cylinder in response to a brake operation, and applies the generated basic hydraulic pressure to wheel cylinders provided at respective wheels connected to the master cylinder by means of a fluid passage at which a hydraulic pressure control valve is provided so that a basic hydraulic braking force is generated at each of the wheels. In addition, the hydraulic brake apparatus applies to the wheel cylinders a controlled hydraulic pressure that is generated by a driving of a pump so that a controlled hydraulic braking force is generated at each of the wheels corresponding to the respective wheel cylinders where the controlled hydraulic pressure is applied. The regenerative brake apparatus causes a regenerative braking force to be generated at the front or rear wheels in response to the state of the brake operation.
According to the aforementioned braking apparatus for a vehicle, a braking force replacement control (i.e., replacement of the regenerative braking force with the controlled hydraulic braking force) is performed for the purposes of ensuring a total braking force required for the wheels by gradually replacing the regenerative braking force with the controlled hydraulic braking force while braking during which at least the application of the regenerative braking force is performed.
The braking force replacement control will be explained with reference to FIG. 9. An upper view in FIG. 9 illustrates a correlation between the braking force and time while a lower view illustrates a correlation between a stroke of a brake pedal and time. At t1, the driver starts depressing the brake pedal in a vehicle in motion. From t1 to t2, the brake pedal is moved at a predetermined depressing speed. From t2 to t3, an amount of depression of the brake pedal is constant. From t1 to t3, the basic hydraulic braking force (portion with a rising diagonal stroke from bottom left to top right indicated as “VB hydraulic pressure” in FIG. 9) and the regenerative braking force (portion with a rising diagonal stroke from bottom right to top left indicated as “regenerative braking force” in FIG. 9) are applied to the wheels. That is, a regenerative cooperative braking is performed.
When the vehicle speed decreases, the regenerative braking force decreases accordingly, which may lead to insufficiency of the regenerative braking force in the total braking force required for the wheels. The shortage of the regenerative braking force is covered or compensated by the controlled hydraulic braking force (portion with a rising diagonal stroke from bottom right to top left indicated as “ESC pressurization in FIG. 9). That is, the braking force replacement control is performed in such a way to be started at t3 and finished at t4.
As illustrated in FIG. 10, when the vehicle speed reaches a predetermined speed (i.e., start replacement vehicle speed) Va1 at t3, the regenerative braking force starts decreasing. When the vehicle speed further decreases to reach a predetermined speed (i.e., end replacement vehicle speed) Vb1, the application of the regenerative braking force is stopped. That is, the braking force replacement control is started when the vehicle speed reaches the predetermined speed Va1 and is stopped when the vehicle speed reaches the predetermined speed Vb1. Accordingly, after a point of t4, the basic hydraulic braking force and the controlled hydraulic braking force are applied to the wheels and finally the vehicle is stopped at t5.
According to the braking apparatus for a vehicle disclosed in JP2006-21745A, while the braking force replacement control is being conducted between t3 and t4, the brake pedal is further depressed or lowered without further action by the driver, i.e., the pedal stroke becomes larger than that obtained during a period between t2 and t3, because of an operation of a pump for applying the controlled hydraulic pressure to the wheel cylinders. As a result, the driver may have an uncomfortable feeling that the brake pedal is further depressed or lowered without his or her further action.
Especially, when comparing the cases illustrated in FIG. 10 and FIG. 11 in which a deceleration of the vehicle is greater than that in FIG. 10, a time period for the vehicle speed to reach the end replacement vehicle speed is shorter in FIG. 11 because of the large deceleration even if the braking force replacement control is started at the same time of t3 with the same regenerative braking force. That is, in FIG. 11, a decreasing rate of the regenerative braking force is large and thus a depressing speed of the brake pedal becomes large regardless of the same amount of the pedal stroke. Accordingly, the driver of the vehicle may have an uncomfortable feeling that the brake pedal is depressed suddenly without further action.
In addition, in the case of replacing the regenerative braking force with the controlled hydraulic braking force, a brake fluid supplied by the pump at the time of replacement start becomes insufficient and thus a start-up delay of the controlled hydraulic braking force may occur, thereby preventing the deceleration that has been achieved before replacement from being retained.
Thus, a need exists for a braking apparatus for a vehicle that can provide an improved brake feeling by achieving a certain depressing speed of a brake pedal during a braking force replacement control. Further, a need also exists for a braking apparatus for a vehicle that can prevent a start-up delay of a controlled hydraulic braking force at the time of replacement start caused by an insufficient brake fluid supplied by a pump for the purposes of retaining a deceleration that has been obtained before the replacement.